I concur that phrasing like "setting the volume to zero" is awkward.
How about:
1.4.1 Audio Turnoff: If any audio plays automatically for more than 3
seconds, either a mechanism is available to pause or stop the audio, or
a mechanism is available to <del>control</del> <ins>reduce or
eliminate</ins> audio volume which can be set independently of the
system volume.
Actually, is *reducing* audio volume necessary (for Level 1). Isn't
merely the ability to *mute* volume enough? And if we do want to
require "setting the volume to zero" -- How is that really different
than pause/stop the audio? If they are all the same, 1.4.1 can be
simplified:
1.4.1 Audio Turnoff: If any audio plays automatically for more than 3
seconds, a mechanism is available to pause, stop, or mute the audio.